Method of making corrosion resistant articles having a bright surface



Patented Apr. 27, I 1954 UNITED STATES METHOD OF MAKING CORROSIONRESIST- ANT ARTICLES HAVING A BRIGHT SUR- FACE No Drawing. ApplicationMay 15,1952, Serial N0. 288,048

3 Claims. 1

This invention relates "to "a method of making corrosion resistantarticles having a bright surface.

Until recently, corrosion resistant articles having bright surfaces, asfor example automobile bumpers and automobile body trimmings, have beenmade by electrodepositing a layer of nickel or copper and nickel on asteel base, and then electrodepositing a thin layer of "chromium such asten millionths of an inch of chromium on the nickel layer. The nickellayer was of a thickness between a minimum of about .0015" and a maximumof about .0035", averaging about .0025". Such articles had a brightsurface and would withstand corrosion in the standard salt spray testfor about 96 hours.

Recently due to the critical shortage of nickel, a government regulationhas issued requiring that the thickness of the nickel layer average notover .001. An article having a nickel layer .001 thick and a layer ofchromium ten millionths of an inck thick 'on top of the nickel layer,will withstand corrosion in the standard salt spray test for only about24 to 48 hours, and accordingly, for many purposes is unsatisfactory.

Schoonmaker and Stockton Patent 2,315,740 owned by the assignee of thisapplication, discloses a method of making corrosion resistant articleswhich do not have bright surfaces. According to one embodiment of thepatent, a layer of nickel is electrodeposited on a steel base, then alayer of zinc is electrodeposited on the nickel, and thereafter thearticle is heat treated to a temperature sufiicient to cause the nickelto alloy with the zinc. In carrying out the heat treating, the articleis usually heated at a temperature of about 700 F., although thistemperature may vary between about 300 F. and a temperaturesubstantially above the melting point of zinc. The patent points outthat nickel and zinc form alloys having different proportions of nickeland zinc, that the gamma and gamma prime alloys contain in theneighborhood of 80% zinc and 20% nickel, that the gamma and gamma primealloys have high corrosion resisting characteristics, and that the bestpractice in carrying out the nickel-zinc alloying heat treatment is toemploy temperatures and times which develop the gamma and gamma primenickel zinc alloys.

While articles made according to the Schoonmaker and Stockton patenthave good resistance to corrosion when subjected to the standard saltspray test, they have a dull surface rather than the bright surfacewhich is required for certain uses. If an article is made according tothe patent, and a layer of chromium is applied to it without firstbuffing the article, the product does not have a bright surface. Thechromium deposit disappears when subjected to salt spray for about 12hours. If the product of the patent is first given a light buffing inorder to remove any excess zinc which may be present and to produce abright surface, and then chromium is electrodeposited on the brightsurface, the surface of the chromium plated article will be bright, butthe article still will only resist the corrosion of the salt spray forabout 12 hours.

We have discovered a method whereby bright surfaces which will withstandthe standard salt spray test for a period of time of the order of hourscan be produce'd'while limiting the average thickness of the nickellayer to less than .001.

In carrying out our-invention, we electrodeposit a layer'of nickel onany metal or alloy base on which nickel can be electrodeposited. Apreferred base material is steel. The nickel layer should have athickness of at least .00005", it being unimportant, except from a coststandpoint, as to What the upper limit of the thickness is. Usually,however, the thickness of the nickel layer is between .00005" and .0015,a preferred thickness for the nickel layer being .0004. A layer of zincis then electrodepo'sited on the nickel layer, the zinc layer being atleast .000005". There is no upper limit to the thickness of the zinclayer except as a matter "of economy. Usually the thickness of the zinclayer is between .00001 and .0005, a preferred thickness of the zinclayer being .0001.

The electrodeposit is then buffed for a time sufficient to cause all ofthe zinc to penetrate into and alloy with the nickel, and. to give abright surface. The buffing may be carried out by the use of an ordinary'cloth bu fiing wheel using an ordinary 'buningcompound such as a limehurling compound. The pressure employed in bufling may be the same as iscommonly used for the builing of nickel. As an example, buffing has beenaccomplished satisfactorily employing a cloth bumng wheel having adiameter of about 14" and a width of about 5", by employing a totalpressure of the wheel rotating at a speed of about 1750 R. P. M.

The extent of buiilng required in order to cause all of the zinc topenetrate into and alloy with the nickel and to "give a "bright surface,can be readily determined by the color 'of the electrodeposit. Thearticle which has been plated with nickel and then with zinc has a whiteor milky color due to the zinc deposit. During buffing, this white colorgradually disappears so that when all of the zinc has alloyed with thenickel and there is no free zinc on the surface, the article has theslightly brown cast or appearance of the nickel. After the article hasbeen buffed to the extent indicated, it is electroplated with chromium,as for example a chromium plate having a thickness of ten millionths ofan inch. An article made in the manner described, employing a nickellayer having a thickness of .0004" and a zinc layer having a thicknessof .0001, buffed as described, and then plated with ten millionths of aninch of chromium, has been found to withstand the corrosion of thestandard salt spray test for a period of time of the order of 100 hours.In contrast to this result, an article plated with .0604" of nickel andthen plated with ten millionths of an inch of chromium, without usingany zinc layer, will withstand the standard salt spray test only forabout 12 hours.

The bufiing employed according to our invention removes some of the zinclayer and causes the rest of it to penetrate into and become alloyedwith the nickel layer. The pressure used and the heat developed duringthe buihng are such that the nickel fiows during the buffing operation.

It is believed that the improved results accomplished according to thepresent invention, as compared with chromium plating an article obtainedaccording to the Schoonmaker and Stockton patent wherein the nickel andzinc are caused to alloy by heating without any buihng action, are dueto the fact that buffing produces a different nickel-zinc alloy fromthat produced by mere heating. For instance, a steel plate waselectroplated with a layer of nickel .00 4" thick, then with a layer ofzinc .0001" thick, and then buffed in the manner described until all ofthe zinc had penetrated into and alloyed with the nickel layer. ihesurface of the buffed article was analyzed and was found to containabout 12% zinc and about 88% nickel. It is believed that the goodadherence of the chromium deposit made according to our method in whichbuffing is employed to cause the zinc to penetrate into and alloy withthe nickel, is due to the .presence of a nickelzinc alloy containing arelatively small proportion of zinc and a relatively large proportion ofnickel and having a slightly brown cast or appearance, as contrastedwith the gamma nickelzinc alloy containing about 80% of zinc and ofnickel and having a blue cast or appearance, produced by heating inaccordance with the Schoonmaker and Stockton patent. In any event,greatly improved results are obtained by following our method since,when a product is made according to the patent and chromium plated andsubjected to salt spray, the chromium disappears when subjected to thestandard salt spray test for a period of about 12 hours.

In carrying out our method, the preferred base to which the nickel isapplied is steel, but the base may be aluminum, copper, brass, bronze,zinc, or any ferrous or non-ferrous metal or alloy on which nickel canbe plated.

We prefer that the article which has been plated with nickel and thenwith zinc, not be heated before buffing. We have carried out a series oftests in which steel plates were coated with nickel and then with zinc.One of these plates was not heated before bufiing. Others were heatedfor seven minutes at temperatures of 350 F., 500 F. and 700 F. We foundthat the plate which was not heated required the least buffing,

and that as the heating temperature before buffing was increased, morebuffing was required in order to produce a surface which, after platingwith chromium, satisfactorily resisted salt spray corrosion.

So far as the thickness of the zinc layer concerned, while a thicknessof .0001" is pre ferred, satisfactory resistance to corrosion has beenobtained with a zinc layer as thin as .000005. No harm is done byemploying a zinc layer greatly exceeding .0001 thick, since it sim-- plymeans that the buffing will have to be continued for a longer time inorder to remove cess zinc and to cause the remaining zinc to penetrateinto and alloy with the nickel, which of course is uneconomical.

The chromium plating can be done by the use of a standard chromiumplating bath operated under standard conditions. It can be a chromiumsulphate bath containing 45 ounces per gallon of C103 and 0.45 ounce pergallon of S04. Zihe plating may be carried out at a temperature of about118 F. employing a cathode current density of about amperes per squarefoot. The thickness of the chromium layer is not criti Since allchromium plating is somewhat porous, it is not relied upon to impartcorrosion re stance to the article but is used simply to prot a brightsurface.

The invention is not limited to the preferred "merit, but may beotherwise embodied or ced within the scope of the following claims.

We claim:

1. The method of making a corrosion resistant article having a brightsurface, which comprises electrodepositing on a metal base a layer ofnickel having a thickness or" at least 00005", electrodepositing on thenickel layer a layer of zinc having a thickness of at least .000005",buffing the deposit for a time sufficient to cause all of the zinc topenetrate into and alloy with the the nickel and produce a brightsurface, and electrodepositing a layer of chromium on the brightsurface.

2. The method of making a corrosion resistant article having a brightsurface, which comprises electrodepositing on a steel base a layer ofnickel having a thickness of .00005"-.0015, electrodepositing on thenickel layer a layer of zinc having a thickness of .0000l-.0005, buffingthe deposit for a time suihcient to cause all of the zinc to penetrateinto and alloy with the nickel and produce a. bright surface, andelectrodepositing a layer of chromium on the bright surface.

3. The method of making a corrosion resistant article having a brightsurface, which comprises electrodepositing on a steel base a layer ofnickel havin a thickness of about .6004, electrodepositing on the nickellayer a layer of zinc having a thickness of about .0001, buffing thedeposit for a time sufficient to cause all of the zinc to penetrate intoand alloy with the nickel and produce a bright surface, andelectrodepositing a layer of chromium on the bright surface.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,315,740 Schoonmaker et a1, Apr. 6, 1943 FOREIGN PATENTSNumber Country Date 474,559 Great Britain Nov. 3, 1937 593,763 GreatBritain Oct. 24, 19 i?

1. THE METHOD OF MAKING A CORROSION RESISTANT ARTICLE HAVING A BRIGHTSURFACE, WHICH COMPRISES ELECTRODEPOSITING ON A METAL BASE A LAYER OFNICKEL HAVING A THICKNESS OF AT LEAST .00000", ELECTRODEPOSITING ON THENIXKEL LAYER A LAYER OF ZINC HAVING A THICKNESS OF AT LEAST .00005",BUFFING THE DEPOSIT FOR A TIME SUFFICIENT TO CAUSE ALL OF THE ZINC TOPENETRATE INTO AND ALLOY WITH THE NICKEL AND PRODUCE A BRIGHT SURFACE,AND ELECTRODEPOSITING A LAYER OF CHROMIUM ON THE BRIGHT SURFACE.